(CBS News) From the bombings at the Boston Marathon, to a fiery blast at a Texas fertilizer plant . . . the victims and their loved ones were ripped from one another in an instant. If only they'd had more time to say goodbye. Lee Cowan reports our Cover Story.

They came without any warning . . . first, the bombs in the bustling city of Boston -- and then, two days later, another blast in the small town of West, Texas.

They seem to have little in common, except the grief they caused. But they do remind us we don't get to set life's clock.

While we may think we'll have a tomorrow to say all the things we want to say, or should have said, what this week proved is that sometimes, that tomorrow doesn't come -- and the things left unsaid could end up one of our greatest regrets.

"You know, just because someone leaves our lives, it doesn't mean that all of the feelings and the emotions and the things that we want to say to them leave, too," said Jackie Hooper. "They just don't disappear."

Hooper has never forgotten a loss that had nothing to do with her -- actress Natasha Richardson. Richardson was skiing in 2009 when she took a fall. Nothing serious, it seemed. Richardson went back to her hotel room with nothing more than a headache.

But two days later she was dead, from an epidural hematoma -- too quick and too unexpected for her family to even say goodbye.

"So that's when I started thinking: what would people say to someone if they had another chance to say it?" Hooper said.

As a young writer Hooper decided to make it her project. She began asking her simple question to just about anyone who would listen. "I started drafting a letter of how I would introduce it to business, schools, retirement homes, jails, and I did it."

Soonher home in Portland, Oregon, was buried in heartfelt letters. Most, she quickly noticed, were written to people who had died unexpectedly.

As they piled up she started a blog, then a book, appropriately titled: "The Things You Would Have Said."

"Dear Melanie, you were too young to die in that car accident . . . "

The letters came from all over the world.

"Please forgive me, I can't forgive myself. Love you. Mom."

They are from mothers who regret fighting with their daughters, from bullies who regret their bullying.

"I know I am better today from the lessons I learned, but still wish I had done better when I was 17 years old."

There are even a few from children -- one bidding farewell to a pet fish.

"Dear Goldie. I miss you so much, that my mom had to buy me a bird and a dog."

All intimate, all important, but for one reason or another were things never said out loud.

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Hooper says the process of putting their regrets down on paper is a form of closure: "There is this feeling of having that final conversation. People even say when they sent it to me, it almost feels like they're sending it to that person that they can't reach. And so it's physically getting it away from inside of them to somebody else. And it can really be healing."

Professor Neal Roese at Northwestern University in Chicago has been studying the emotion of regret for 20 years. He agrees, if channeled right, it can be beneficial.

"Regret can serve a healthy purpose, if we listen to a message or draw an insight, but then move on and focus on the future," Roese told Cowan.

How many of us took extra special care to make sure we kissed our kids goodnight this week? Or called a friend or a grandparent just to say hello?

It's the missed opportunities -- when we could have acted but didn't - that sometimes are the most haunting.

"Things that are left undone tend to be more powerful, and longer lasting," said Roese. "Especially when we think about worlds left unsaid, things that we wished we had told a loved one before it was too late. These tend to last a long time because they invite our imagination to fill in the blanks in enumerable ways.

Take 80-year-old Ernest Waxman, who happens to be Jackie Hooper's grandfather. He's a Holocaust survivor -- a survivor only because of the Gabriels, his Christian neighbors in Berlin, who hid him from the Nazis during Kristallnacht, an infamous night of anti-Jewish rioting and murders.

"You were old enough to know what was going on?" asked Cowan.

"Oh, listen. I think any 10-year-old kids would know," Waxman said.

He eventually fled to safety in Sweden, got married, and came to the U.S. But ever since the war he carried with him a sense he had never thanked that family quite enough.

"You were angels who put themselves in danger in order to help save my family," he wrote. "The memory of what you have done for me remains unchanged."

Waxman said writing the letter made him feel better: "Absolutely, absolutely. It has been on my mind all those years, you know, and I finally had a way to express myself."

Hooper said, "I think anytime you have the chance to tell somebody that they've made an impact on your life, you need to take that opportunity, and you need to make the opportunity."

"She had very old-fashioned cat-eye glasses, and she wore big shoes that looked like bricks," Dodds said. "I, like, clomped around the halls like, I'm Mrs. Musselman with my big orthopedic shoes!, you know. Like, I'm Frankenstein, and I made fun of her. I don't know why when I was younger I thought that that was appropriate, you know?"

But out of childhood regret came this sobering letter:

"Please accept my apology for being an unappreciative student in your class in 10th grade. I use the concepts you elucidated for me every day of my life. Thank you for persevering with us all, and teaching me, despite my resistance. Sincerely, Debby B."

Mrs. Musselman died before learning what Debby grew up to be: A teacher.

"I always thought maybe I would see her at a reunion," Dodds said. "Or I would be at a alumni function or something like that, and I would see her."

But that never happened.

Time has an uncomfortable way of sneaking up on all of us.

We hope this week will be better than last -- but the fact is, we're never really sure.

"The big thing for me is remembering that final conversation you have with people," Hooper said. "I know that if my husband's busy getting ready upstairs for work, and he runs out the door because he's late, and he just says 'Bye,' he's pretty sure that within a couple of minutes he'll get a text or a phone call saying, 'I love you.' Because those final moments are very clear in my mind now.